Production of thermally stabilized polyester

ABSTRACT

High molecular weight linear condensation polyesters are stabilized against deterioration by heat by reacting the polyester in molten form with an epoxy compound having 5 to 25 carbon atoms in the molecule and selected from the group consisting of ##STR1## where R represents the radical remaining after removal of the carboxyl group from a monocarboxylic acid, R 1 , R 2 , R 3  and R 4  represent hydrogen or hydrocarbon radicals, and n is an integer that can be 0 to 3.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 819,654, filed July 27, 1977.

This application is related to our copending application Ser. No.806,988 filed June 16, 1977.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a heat stable fiber forming polyester and to anew and novel process for preparing it. More particularly, thisinvention relates to an improved linear high molecular weight heatstable polyester especially suitable for preparing fibers which haveexcellent resistance to degradation when utilized in commercialarticles, such as tires, industrial belting, etc. wherein a high degreeof heat is built up during use.

2. Description of the Prior Art

High molecular weight polyethylene terephthalate fiber formingpolyesters are well known. They are prepared commercially either by theester interchange reaction between dimethyl terephthalate and ethyleneglycol or by the direct esterification process wherein terephthalic acidis reacted directly with ethylene glycol. These products and processesare well documented in U.S. patents, such as U.S. Pat. Nos. 2,465,310;3,050,533; 3,051,212; 3,427,287 and 3,484,410 which cover not only thebasic products and processes but many improvements thereon.

Polyethylene terephthalate fibers and cords are known to exhibitexcellent dimensional stability, that is, low extension or growth duringservice, as well as to have a high resistance to thermal degradation;however, in pneumatic tires and industrial belts under high speedconditions under heavy loads, loss of tensile strength is experienceddue to high temperature conditions emanating under such conditions.Efforts to remedy this problem have all too often been ineffective. Mostresearch in this field has been directed to producing a high molecularweight linear polyester having a low content of free carboxyl groups.The following patents are pertinent.

U.S. Pat. No. 3,051,212 to William W. Daniels relates to reinforcedrubber articles and to textile cords and fibers for reinforcing sucharticles. This patent discloses that a linear terephthalate polyesterhaving a concentration of free carboxyl groups of less than 15equivalents per million grams may be prepared in a number of differentways. One effective procedure is to treat the filaments, after they havebeen formed, with a chemical reagent which reacts with and "caps" thefree carboxyl group. One such agent is diazomethane.

U.S. Pat. No. 3,627,867 to Eckhard C. A. Schwartz discloses a processand apparatus for melt spinning high molecular weight polyethyleneterephthalate into high-performance fibers under conditions which reducethe normally high viscosity of such polyester. Ethylene oxide or otherlow-boiling oxirane compound is injected under pressure into moltenpolyester before it is fed to the metering pump of the melt-spinningmachine. The fibers are characterized by low free-carboxyl content andfreedom from voids which might be expected from injection of thevolatile material.

U.S. Pat. No. 3,657,191 to Rudolph Titzmann et al. is directed to aprocess for the manufacture of linear polyesters having an improvedstability with respect to compounds with active hydrogen. Polyesters ofthis type are obtained by reacting polyesters with ethylene carbonatesor monofunctional glycidyl ethers. The reaction is first carried outwithin a temperature range lying 10° to 60° C. below the softening pointof the polyester and is then terminated during the melting andmelt-spinning process.

U.S. Pat. No. 3,869,427 to Robert W. Meschke et al. discloses a processof preparing polyester filaments having low free-carboxyl-group contentswhich give superior performance in pneumatic tires and other reinforcedrubber articles where heat-degradation is a problem. Reduction of freecarboxyl groups is achieved by mixing with the molten polyester, priorto melt-spinning, 1,2-epoxy-3-phenoxypropane or1,2-epoxy-3-n-hexyloxypropane.

U.S. Pat. No. 4,016,142 to William Alexander et al. disclosespreparation of a fiber-forming polyester wherein the number of freecarboxyl end groups present in the polymer may be reduced by adding tothe polymerized polyester a glycidyl ether which reacts with thecarboxyl end groups present to form free hydroxyl end groups.

Although the above-identified patents directed to stabilized polyestersare of major interest, certain of the proposed polyester modifiers areknown to be highly toxic and/or hazardous to use on commercial scale.Moreover, we have found that the others are realtively less effective interms of reducing the carboxyl end group concentration of the polyester.Accordingly, we have carried out considerable research in this field tosolve or mitigate the long-standing problem of producing high molecularweight polyester stabilized against deterioration under high temperatureoperating conditions.

SUMMARY OF THE INVENTION

The present invention relates to an improved high molecular weight heatstable polyester and to a novel process for preparing it. The inventionfurther provides polyester fibers which have excellent resistance tothermal degradation when utilized in commercial articles, such as tires,industrial belting, etc. wherein a high degree of heat is built upduring use.

In accordance with the above objects, it has now been discovered that animproved heat stable fiber forming linear condensation polyester isobtained by incorporating therein a stabilizing amount of a stabilizercomprising an epoxy compound having 5 to 25 carbon atoms in the moleculeand selected from the group having the formulae: ##STR2## where Rrepresents the radical remaining after removal of the carboxyl groupfrom a monocarboxylic acid, R₁, R₂, R₃ and R₄ represent hydrogen orhydrocarbon radicals, and n is an integer that can be 0 to 3. This novelpolyester is obtained without undue difficulties in the processingthereof and the additive is compatible with other additives that may bedesirable for specific uses. Preferably, the polyester in molten form isreacted with the epoxy compound, whereby the resulting thermallystabilized polyester has a free carboxyl concentration of less than 15gram equivalents of carboxyl groups per 10⁶ grams of polyester.

The epoxy compounds useful as stabilizers in the present invention areknown compounds or are readily prepared by known procedure. Inparticular, amides of epoxyamines and carboxylic acids and thepreparation thereof are described in U.S. Pat. No. 2,730,531 to GeorgeR. Payne et al., and U.S. Pat. No. 2,772,296 to Albert C. Muellerdiscloses a process for preparing epoxy esters, e.g., glycidyl benzoateis prepared by reacting benzoic acid with epichlorohydrin. Thecorresponding glycidyl thiobenzoate may be prepared from thiobenzoicacid.

The preparation of the improved polyester can be carried out bycondensing an aromatic dicarboxylic acid, preferably terephthalic acid,and/or the lower alkyl ester thereof with a glycol containing 2 to about10 carbon atoms per molecule under direct esterification and/orester-interchange conditions. A stabilizing amount of theabove-described stabilizer may be incorporated before, during or afterpolycondensation of the polyester. Preferably, the stabilizer is addedto the molten polyester after the final polycondensation of the polymer.

The esterification of the aromatic dicarboxylic acid and/or the loweralkyl esters thereof and the glycol can start at a temperature as low as200° C. and range up to 300° C. and at atmospheric and superatmosphericpressures ranging up to 500 psig. The reaction, either the directesterification or ester-interchange is carried out in the absence ofoxygen-containing gas. Preferably, the reaction temperature ranges fromabout 230° C. to about 280° C. and at a pressure ranging from about 50to 250 psig. The reaction time will vary depending upon the reactiontemperature and pressure. The glycol is reacted with the aromaticdicarboxylic acid and/or the lower alkyl ester thereof in an amountranging from about 1 to about 3 mols of glycol per mol of acid. Theamount of said epoxy compound added as stabilizer ranges generally from5 to 70 gram mols of epoxy compound per 10⁶ grams of the polyester.Preferably, 10 to 50 gram mols of epoxy compound is added per 10⁶ gramsof the polyester.

Other additives can be added to the polymer with complete compatibilitytherewith to control or tailor the reactions in order to obtain requiredcharacteristics of the final polymer for specific end uses. Many suchadditives are known and utilized to control dyeing, static, luster,flammability, light stability, brightness, etc.

The polycondensation of the esterification product obtained by thedirect esterification or ester-interchange reaction between aromaticdicarboxylic acid or lower alkyl ester thereof with a glycol is usuallycarried out at a reduced pressure which can be as low as 0.1 torr and atemperature in the range of from about 260° C. to about 300° C. Thispart of the reaction is carried out under these conditions for periodsof about 1.0 to about 10 hours and preferably from about 2 to about 6hours until a polymerized polyester product of the required molecularweight as determined by viscosity or other convenient physical measuresis obtained. The duration of such periods depends upon the variousprocess polymerization conditions such as pressure and temperatureprofiles, ingredient mol ratios, surface generation conditions, catalysttype and concentration, any additives utilized, requisite viscosity,etc. Polycondensation is generally continued until the resultantpolyester has an intrinsic viscosity in 60 percent phenol-40 percenttetrachloroethane mixture of about 0.6 to 1.0, preferably 0.8 to 0.95.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As previously mentioned, the present invention further providespolyester fibers which have excellent resistance to degradation whenutilized in commercial articles, such as tires, industrial belting, etc.wherein a high degree of heat is built up during use. Accordingly, onepreferred embodiment of this invention may be briefly stated as follows:

In the preparation of fibers particularly useful in reinforced rubberarticles such as pneumatic tires and industrial belts, from highmolecular weight linear terephthalate condensation polyester, the methodof providing a reduction in the free carboxyl content of the polyesterto a carboxyl concentration of less than 15 gram equivalents per 10⁶grams of polyester which comprises adding to the polyester after finalpolycondensation of the polyester a thermally stabilizing amount of astabilizer comprising an epoxy compound selected from the group havingthe formulae: ##STR3## where R represents the radical remaining afterremoval of the carboxyl group from a monocarboxylic acid and n is aninteger that can be zero or one, said epoxy compound having 5 to 20carbon atoms in the molecule. Preferably, 10 to 50 gram mols of saidepoxy compound are added to the polyester per 10⁶ grams of thepolyester.

The following examples are illustrative of embodiments of the presentinvention but are not to be constured as limiting the invention in anyway. The ingredient parts are expressed as stated in the examples.

EXAMPLE 1

About 41.5 pounds per hour of terephthalic acid, 27.9 pounds per hour ofethylene glycol, 65 grams per hour of diisopropylamine and 16 grams perhour of antimony acetate are continuously fed to a paddle mixer wherethey are converted to a paste. The paste mixture is then pumped from themixer by a feed pump to the inlet of a circulating pump. The pastemixture is pumped with 40 parts by weight per part of paste mixture ofrecirculating mixture by the circulating pump through a multiple tubeand shell heat exchanger where it is heated to 260°-270° C. Afterleaving the heat exchanger, the mixture enters an esterification reactorwhich is maintained at 260°-270° C. by conventional heating means, and90 psig. pressure by means of an automatic vent valve. The recirculatingmixture leaving this reactor is split, with part being returned to theinlet of the circulating pump where it is combined with fresh paste andpart flowed to a series of three reactors where further esterificationtakes place at 270°-275° C. Total esterification time is about 3 hours.Following esterification the reaction mixture is fed into apolycondensation reactor operating at 275° C. and 30 torr pressure, witha residence time of 60 minutes. The resulting polyester polymer is fedto a polycondensation reactor operating at 275° C. and 2 torr pressure,with a residence time of 120 minutes. Then, the polyester polymer isprocessed in a final polycondensation at 278° C. and 0.5 torr pressurefor 130 minutes. The polyester polymer melt at about 278° C. is pumpedfrom the final polycondensation reactor by means of a screw pump andconducted to gear pummps for transfer to a spinning machine wherepolymer temperature is increased to about 300° C. Between the screw pumpand the gear pump, 0.255 pound per hour of N-(2,3 epoxypropyl)benzamideis added to the polyester polymer as stabilizer and intimately mixedwith the polymer by means of a conventional stationary mixer. Thepolyester polymer is reacted with the N-(2,3 epoxypropyl)benzmide for3-20 minutes at about 278° to 300° C. until the polymer is spun at therate of 48 pounds per hour through a 192 hole spinnerette. Yarn iscontinuously spun and drawn to form 1300 denier, 192 filament yarn. Theundrawn yarn from the spinnerette has an intrinsic viscosity of 0.80 to0.90 dl. per gram and about 12 gram equivalents of carboxyl end groupsper 10⁶ grams of polyester. The drawn yarn has 15.9 percent ultimateelongation and 8.5 grams per denier tensile strength. The drawn yarnretains 87 percent of its strength after exposure to pure ammonia gasfor 3 hours at 150° C. This test shows that the yarn is very stable toboth heat and ammonia, which is indicative of a good tire yarn. Thedrawn yarn is overfinished with a lubricating composition, twisted into3 ply, 9 t.p.i. tire cord, woven into a fabric, dipped in a blockeddiisocyanate-epoxide emulsion, stretched at 420° F., dipped in aresorcinol-formaldehyde-vinyl pyridine polymer emulsion, stretched at440° F., and calendered with rubber to make rubberized fabric for tirebuilding. Tires made with this fabric are characterized by excellentdurability when run on the wheel test stand.

Similar results are obtained when equivalent amounts of N-(2,3epoxypropyl)-stearamide, N-(epoxyethyl)-benzamide, glycidyl benzoate orS-(glycidyl)-thiobenzoate are used in place of the N-(2,3epoxypropyl)-benzamide.

EXAMPLE 2

This example demonstrates the use of 4-dimethylaminopyridine as acatalyst to accelerate the reaction of polyethylene terephthalate withan epoxy compound of the present invention.

About 48 pounds of polyethylene terephthalate chips having an intrinsicviscosity of 0.95 are mixed with 0.255 pound of N-(2,3epoxypropyl)-benzamide and 0.01 pound of 4-dimethylaminopyridine bytumbling in a can. The mixture is then melted and spun at about 300° C.through a 1-inch extruder into 48 filament yarn which is plied and drawnat a draw ratio of 6.05 to 1 into 1300 denier, 192 filament yarn. Theundrawn yarn from the spinnerette has an intrinsic viscosity of 0.84 and9 equivalents of carboxyl end groups per 10⁶ grams. The drawn yarn has14.5 percent ultimate elongation and tensile strength of 8.4 grams perdenier. The drawn yarn retains 90 percent of its strength after exposureto pure ammonia gas for 3 hours at 150° C. This yarn is converted intotire cord as in the first example. This cord is characterized as havingexcellent fatigue and durability properties.

EXAMPLE 3

Example 1 is repeated except that 0.51 pound per hour of N-(2,3epoxypropyl)-benzamide is added to 48 pounds per hour of the polyesterpolymer. The undrawn yarn from the spinnerette has an intrinsicviscosity of 0.80 to 0.90 and about 5 equivalents of carboxyl end groupsper 10⁶ grams of polyester. The drawn yarn has 17.3 percent ultimateelongation and tensile strength of 8.1 grams per denier. The drawn yarnretains 96 percent of its strength after exposure to pure ammonia gasfor 3 hours at 150° C.

EXAMPLE 4 (Comparative)

Example 1 is repeated except that no N-(2,3-epoxypropyl)-benzamide isadded to the polyester polymer. The undrawn yarn from the spinnerettehas an intrinsic viscosity of 0.80 to 0.90 and 30 equivalents ofcarboxyl end groups per 10⁶ grams of polyester. The drawn yarn has 16.7percent ultimate elongation and tensile strength of 8.2 grams perdenier. The drawn yarn retains only 59 percent of its strength afterexposure to pure ammonia gas for 3 hours at 150° C. These data incomparison with the data of Examples 1-3 demonstrate the beneficialeffect respecting number of carboxyl end groups and strength retentionof the polyester yarn of adding the stabilizer compound of the presentinvention.

EXAMPLE 5 (Comparative)

To demonstrate the criticalness of using the particular epoxy compoundsof the present invention, Example 1 is repeated except that equivalentamounts of 4-methoxyphenyl-2,3-epoxypropyl ether, phenyl-2,3-epoxypropylether or C₈ +C₁₀ n-alkyl epoxypropyl ethers are used in place of theN-(2,3 epoxypropyl)-benzamide of Example 1. The undrawn yarn from thespinnerette has an intrinsic viscosity of 0.85 and 27-29 equivalents ofcarboxyl end groups per 10⁶ grams of polyester. The drawn yarn issimilar in properties to that produced in comparative Example 4.

Although we do not wish to be bound by any theory as to the mechanism ofthe present invention, we believe that the activity of the epoxycompounds of the present invention relates to the structure of theoverall molecule and particularly to the position of the carbonyl oxygenwith respect to the epoxy oxygen.

The proposed mechanism for the present invention is illustrated belowwith a typical epoxy compound of the invention, glycidyl benzoate.##STR4##

We have found that optimum activity of the epoxy compounds of thepresent invention results when there are 3 to 4 atoms between the epoxyoxygen and the carbonyl oxygen. This corresponds to 6 or 7 atoms in theabove-described cyclic intermediates.

We claim:
 1. A thermally stabilized high molecular weight linearterephthalate condensation polyester having incorporated therein athermally stabilizing amount of a stabilizer consisting of an epoxycompound having 5 to 25 carbon atoms in the molecule and selected fromthe group having the formula ##STR5## where R represents the radicalremaining after removal of the carboxyl group from a monocarboxylicacid, R₁, R₂, and R₃ represent hydrogen or hydrocarbon radicals, and nis an integer that can be 0 to 3, said thermally stabilized polyesterhaving a free carboxyl concentration of less than 15 gram equivalents ofcarboxyl groups per 10⁶ grams of polyester.
 2. The polyester of claim 1wherein the linear terephthalate condensation polyester is polyethyleneterephthalate.
 3. The polyester of claim 1 wherein 5 to 70 gram mols ofsaid epoxy compound is incorporated per 10⁶ grams of the polyester. 4.The polyester of claim 1 wherein 10 to 50 gram mols of said epoxycompound is incorporated per 10⁶ grams of the polyester.
 5. Thepolyester of claim 1 additionally containing a catalytic amount of4-dimethylaminopyridine.
 6. In a process for the preparation of a highmolecular weight linear terephthalate condensation polyester whereinterephthalic acid is reacted with a glycol containing 2 to 10 carbonatoms per molecule under esterification conditions and the resultingesterification product is polycondensed, the improvement which comprisesproviding a reduction in the free carboxyl content of the polyester to acarboxyl concentration of less than 15 gram equivalents per 10⁶ grams ofpolyester by adding to the polyester after final polycondensation of thepolyester a thermally stabilizing amount of a stabilizer comprising anepoxy compound having 5 to 25 carbon atoms in the molecule and selectedfrom the group having the formula ##STR6## where R represents theradical remaining after removal of the carboxyl group from amonocarboxylic acid, R₁, R₂, and R₃ represent hydrogen or hydrocarbonradicals, and n is an integer from 0 to
 3. 7. The process of claim 6wherein the linear terephthalate condensation polyester is polyethyleneterephthalate.
 8. The process of claim 6 wherein 5 to 70 mols of saidepoxy compound is incorporated per 10⁶ grams of the polyester.
 9. Thepolyester of claim 6 wherein 10 to 50 gram mols of said epoxy compoundis incorporated per 10⁶ grams of the polyester.
 10. The process of claim6 wherein said epoxy compound is added to the polyester together with acatalytic amount of 4-dimethylaminopyridine.
 11. In the preparation oftire yarn from a linear polyethylene terephthalate condensationpolyester by melt spinning, the method of providing a reduction in thefree carboxyl content of the polyester to a carboxyl concentration ofless than 15 gram equivalents per 10⁶ grams of polyester which comprisesadding to the polyester after final polycondensation of the polyester athermally stabilizing amount of a stabilizer comprising an epoxycompound having 5 to 25 carbon atoms in the molecule and selected fromthe group having the formula ##STR7## where R represents the radicalremaining after removal of the carboxyl group from a monocarboxylicacid, R₁, R₂, and R₃ represent hydrogen or hydrocarbon radicals, and nis an integer from 0 to 3.